Where does it say they were Mary’s children?
This gate shall be opened to no man, for the Lord only will go in by that gate, and again go out, and it shall be shut forever (Ezekiel 44;2).
It is a consistent understanding by all of the Great Theologians of Christendom before 1517 that Christ entered into the womb of the Virgin Mary by the power of the Holy Spirit and and was born (go out) and took his Human Nature from her. After the Incarnation of Christ, it (Mary’s womb) was shut forever.
And again, those brothers of Jesus were never understood to be Mary’s children. James the so called brother of the Lord is actually the Son of another Mary. None of those “brothers of the Lord” are ever called Mary’s child, Christ is several times (John 2:1, Acts 1:14). THe Mark 6:3 you cite, James and Joseph (Joses) were called brothers of the Lord and their Mothers name was Mary. In MT 27:56, we see there were many woman, Mary Magdalene and Mary the Mother of James and Joseph, and the mother of the sons of Zebedee. Is this Mother of James and Joseph Mary the Mother of Christ? I don’t think so.
Look at several other Crucifixion and Resurrection accounts. In MT 28:1 we see “After the sabbath, as the first day of the week was dawning... Mary Magdalene, JOanna and the other Mary (why not Mary the Mother of CHrist). In Luke 24:10 we see another more detailed account and find Mary Magadelene again names, Joanna and Mart the Mother of James (again, not Mother of Christ.
Mark 15:40-47 we see the text indicating that there were also woman looking from the distance, among them were Mary Magadelene, Mary the Mother the younger James, Joses and Salome. Mark 16:1-2 we see Mary Magadelne and Mary the Mother of James and Salome bringing spices.
In reading these texts, the Children are used to identify which “MARY” is being referred to. There 2 being referred to here, Mary Magdalene and the other Mary, The Mother of James, Joses and Salome and this Mary ain’t the mother of Jesus.
John’s Gospel indicates Mary the Mother of Christ was present at the Crucifixion and Jesus committed his Mother to John (why not James, Joses and Salome, if they were Mary, the Mother of Jesus children as well).
John’s Gospel tells us in Verse 25 “Near the Cross of Jesus stood HIS mother and his mother’s sister, Mary the wife of Clopas and Mary Magdala.
Now we now have 2 other Mary’s identified along side Mary the Mother of Christ and as Matthew, Mark and Luke all tell us, this “other Mary” is the Mother of James, Joses, and Salome. Since all 4 Gospels are consistent with there being THREE Mary’s, 1)Christ Mother, 2)Mary Magadelene and 3)The Other Mary, the Mother of James, Joses and Salome, this other Mary seems to be Mary (Christ’s Mother’s) sister, who was the wife of Clopas. Again, look at the text of John 19:25 and I am assuming there are only 3 Mary’s there and Mary’s sisterhood is connected to Mary either by virtue of her being the wife of Clopas (Joseph’s brother) or maybe she was a close relative to Mary the Mother of Christ and called “sister” as the bible used the term brother and sister to describe close relatives including uncles and cousins.
Regardless, James, Joses and Salome are indeed relatives of Christ, but not brothers and sisters of Christ in the sense they were the Virgin Mary’s children.
The Bible does not support it nor does any orthodox theologian of the early Church.
To borrow from the 1970’s song from the rock band queen, another protestant heresy bites the dust.
Why did early Christians think that Jesus was conceived and died on the same time?
It is always vitally important bear cultural, lingual, and historical context well in mind when considering the Bible, to avoid such misunderstandings as this. You can't just look and say, "my English Bible that was translated from Greek that was used to record things said in Aramaic says "brother," so it must mean exactly what "brother" is understood to mean today, thousands of years of cultural and lingual changes later."
Till she brought forth her firstborn son—From these words Helvidius and other heretics most impiously inferred that the blessed Virgin Mary had other children besides Christ; but St. Jerome shews, by divers examples, that this expression of the Evangelist was a manner of speaking usual among the Hebrews, to denote by the word until, only what is done, without any regard to the future. Thus it is said, Genesis 8. 6 and 7, that Noe sent forth a raven, which went forth, and did not return till the waters were dried up on the earth. That is, did not return any more. Also Isaias 46. 4, God says: I am till you grow old. Who dare infer that God should then cease to be: Also in the first book of Machabees 5. 54, And they went up to mount Sion with joy and gladness, and offered holocausts, because not one of them was slain till they had returned in peace. That is, not one was slain before or after they had returned. God saith to his divine Son: Sit on my right hand till I make thy enemies thy footstool. Shall he sit no longer after his enemies are subdued? Yea and for all eternity. St. Jerome also proves by Scripture examples, that an only begotten son, was also called firstborn, or first begotten: because according to the law, the firstborn males were to be consecrated to God; Sanctify unto me, saith the Lord, every firstborn that openeth the womb among the children of Israel, etc. Ex. 13. 2.